In a typical integrated circuit (IC) chip, the input, output, power supply and other terminals of the circuit are formed by metalized contacts, usually deployed along the margins of the circuit pattern. The outline of the chip is often square, and the marginal locations of the contacts depend on the circuit configurations and the available marginal space. In some instances the contacts may lie in a uniform row or rows along the margins.
For the purpose of testing any type of integrated circuit patterns, before the application of leads to connect the contacts to other components, various types of test probe cards have been developed. The most commonly used probe card consists of a printed circuit board having a circular opening or port to provide access to contacts on an IC chip. This opening is surrounded by conductive probes connected to terminals on the card which, in turn, are connected to test equipment appropriate to the circuit. The number of probes in the opening determines the maximum capacity of the probe card.
Structures used to assemble typical test probe cards consist of a base containing a downward-pointing funnel. A number of probes 22 are laid in a circular manner around the base with the probe tips pointing down towards the center of the funnel, as shown in FIG. 1A. Due to the curvature of the funnel surface even probes lying next to each other, e.g., probes 22A and 22B at a fan out angle .theta.=0 have different inclination angles .beta..sub.1, and .beta..sub.2 as indicated in FIG. 1B. In general, probes at a fan out angle .theta. will also have a different inclination angle .beta..
After probes 22 are adjusted on the funnel plane, a layer of epoxy is laid over them, as shown in FIG. 3. The epoxy is then hardened by heating and the structure of probes and epoxy is attached to a planar insulation card having a printed circuit and a port.
Since the probes are attached to the probe card in a circular manner, they touch contacts 106 located on an IC chip 105 at different fan out angles .theta., as illustrated in FIG. 2. A probe located in the center of a row of probes touches a contact located in the center of a row of contacts directly, at 0.degree.. However, a probe located at the end of a row of probes requires a steeper fan out angle .theta. to reach a contact located at the end of a row of contacts, in the corner of the chip.
Since the probes are laid on an inclined assembly structure with the probe tips pointing downwards, as shown in FIG. 1A, they will form an inclination angle .beta. with the plane of the IC chip. Since the probes are laid on a non-planar surface, the inclination angle of each probe will vary greatly (see FIG. 1B). In general, probes at the end of the row will have a smaller inclination angle than the probes at the center of the row. The disparity in the inclination angles at which the probes are laid will cause the force between the probe tips and contacts of the IC chip to vary over a wide range.
This disparity also causes deformation of the probe tips when they are placed in the holes of the Mylar sheath. The bend angle of the tip changes and alignment of the probe tips suffers. The Mylar may also be dislodged or deformed by the probe tips, which could cause the alignment to suffer. Often if the inclination angles of the probes differ, the probe tips may even pop out of the Mylar holes, again causing problems with tip alignment and planarity. These events may cause differences in the force at which the probe tip touches the contact and prevent uniform scrub length.
When the probes touch the contacts on the chip, the probe tips scrub the contact surfaces to remove the oxide film, and thereby establish electrical contact, as shown by scrub mark 100 of FIG. 2. Disparity in the inclination angles of the probes will cause non-uniform scrub marks. In addition, because of fan out angles, some of the angles at which the probes touch the contacts are quite large (.about.25.degree.-45.degree.), and the scrubbing motion causes the tips to go beyond the contacts to invade the surface of the chip, as shown by scrub mark 102 of FIG. 2. These scrub marks damage the functionality of the IC chip by destroying the insulating passivation layer.
Since all of the contacts in an IC chip lie in a common plane and must be simultaneously engaged in order to carry out testing, it is essential that all probe tips lie in a plane parallel to the common IC plane. Consequently, a fundamental requirement for a probe card is planarization of the probe tips. After the probes have been adjusted to assume their proper angles, an epoxy is poured over the array of probes so as to embed them at their assigned angles and planes in the epoxy, as illustrated in FIG. 3. The epoxy is not contained, however, and often flows around the probes, dislodging them. Misalignment of the probes occurs so they are no longer at the correct angles or in the correct plane, as seen in FIG. 4. Thus a uniform contact force becomes impossible to achieve.
Because the epoxy flow is uncontrolled, it may run down the length of the probe, as shown in FIG. 3. The result is variance in the amount of probe exposed, also called beam length and indicated by reference L. One will thus get some probes with beam lengths L that are shorter or longer than others. The force at which a probe touches a contact on an IC chip depends to a large extent on the beam length and thus uniform beam lengths are crucial.
With a view to providing a test probe assembly that has uniform and consistent scrubbing characteristics, the Evans U.S. Pat. No. 4,599,599 discloses a structure in which a circular array of probes, all lying in a horizontal plane, are supported on a mounting ring surrounding a circular port in a card. The probes, converge toward the central region of the port below which is the chip to be tested, with the slope angle between each probe tip section and the surface of the chip uniform. Despite the fact that this probe card minimizes the problem of probes aligned on different planes, it still retains the fan out angles for probes touching contacts at the corners of the chip and the resulting scrubbing problem.
The Evans U.S. Pat. No. 4,719,417 discloses a structure for a multi-layered test probe assembly, which features two radial arrays of probes to test an exceptionally large number of contacts on an IC chip. It also retains the fan out angles for probes touching contacts at the corners of the chip and the resulting scrubbing problem.